1. Field of Invention
The present invention relates in one aspect to a method of creating a link from a first piece of compiled code to a second piece of compiled code, and to a method of compiling code. It relates in another aspect to methods of and apparatus for examining memory in a computer system to allow a section of compiled code to be deleted, and to a method of and apparatus for deleting compiled code in a computer system, in particular where there may be a link between sections of compiled code. The invention has particular (but not exclusive) application to a self-modifying multi-threaded environment. In a preferred embodiment, the invention relates to multi-threaded fragment patching.
2. Description of Related Art
In recent years, there have been developments in programming languages towards what is known as an object-oriented language. In these developments, concepts are regarded as ‘objects’, each carrying with it a set of data, or attributes, pertinent to that object, as well as information relating to so-called ‘methods’, that is functions or sub-routines, that can be performed on that object and its data. This is well known to those skilled in the art of computing and/or programming.
The advent and rapid advancement in the spread and availability of computers has led to the independent development of different types of systems, such as the IBM and IBM-compatible PC running IBM-DOS or MS-DOS or MS-Windows applications, the Apple Macintosh machines 3 running their own Apple System operating system, or various Unix machines running their own Unix operating systems. This proliferation of independent systems has led to useful applications being available only in one format and not being capable of running on a machine for which the application was not designed.
Under such circumstances, programmers have devised software which ‘emulates’ the host computer's operating system so that a ‘foreign’ application can be made to run successfully in such a way that, as far as the user is concerned, the emulation is invisible. In other words, the user can perform all of the normal functions of say a Windows-based application on a Unix machine using a Unix-based operating system without noticing that he is doing so.
A particularly notable product of this type is that developed by Insignia Solutions of High Wycombe, GB and Santa Clara, Calif., USA and known under the name ‘SoftWindows 2.0 for Powermac’. This software enables a physical Macintosh computer to emulate a PC having an Intel 80486DX processor and 80487 maths co-processor plus memory, two hard disks, IBM-style keyboard, color display and other features normally found on recent versions of the PC-type of computer.
Furthermore, there is an ever-increasing demand by the consumer for electronics gadgetry, communications and control systems which, like computers, have developed independently of one another and have led to incompatibility between operating systems and protocols. For example, remote-control devices for video players, tape players and CD players have similar functions, analogous to ‘play,’ ‘forward,’ ‘reverse,’ ‘pause,’ etc, but the codes for transmission between the remote control, or commander, operated by the user may not be compatible either between different types of equipment made by the same manufacturer or between the same types of equipment made by different manufacturers. There would be clear benefits of having software within the equipment which can produce for example the correct ‘play’ code based upon a ‘play’ command regardless of the specific hardware used in the equipment. Such software is commonly known as a ‘Virtual Machine.’
Other uses and applications are legion: for example, set-top boxes for decoding television transmissions, remote diagnostic equipment, in-car navigation systems and so-called ‘Personal Digital Assistants.’ Mobile telephones, for instance, can have a system upgrade downloaded to them from any service provider.
Emulation software packages tend to have certain features in common, notably that they are not general purpose but are dedicated. They are of most benefit in rapid development areas and have a distinct advantage in enabling manufacturers to cut costs. In particular, they can divorce software from the physical machine, i.e., the effect of the software in the physical machine can be altered by the emulating software without having to go into the machine's native software to implement those changes.
The specific object-oriented language used in some of the implementations described later is that known as Java (registered trade mark to Sun Microsystems Corporation). Some of the following implementations will enable Java to be used in smaller devices than is currently possible because of the improved performance and/or reduced memory footprint. Future uses projected for embedded software (virtual machines) include computers worn on the body, office equipment, household appliances, and intelligent houses and cars.
While it is recognised that there are clear advantages in the use of virtual machines, especially those using object-oriented languages, there are naturally areas where it is important and/or beneficial for some of the operations that are carried out within the system to be optimised.
These may include reducing the memory requirement, increasing the speed of operation, and improving the ‘transparency’ of the system when embedded in another system. One of the principal aims of the invention described herein is to provide a Virtual Machine which is optimised to work as quickly as possible with in a memory constraint of, for example, less than 10, 5, 2 or even 1 Mbyte. Such a constraint is likely to be applicable, for example, to electronics gadgetry and other equipment where cost (or size) is a major constraint.
A self-modifying environment may be one in which sections of compiled code are created and deleted dynamically during execution. Such an environment is described in Agent's Reference No. 1 of this specification. A multi-threaded environment is one in which several processes, or threads, operate asynchronously in the same workspace.
In a self-modifying environment there may be situations in which a link must be made between a first section of compiled code and a second section of compiled code that is located elsewhere in the workspace, to enable execution to transfer between the two sections of code. The process of transferring execution from one piece of code to the other generally involves a number of steps, including putting the address of the first piece of code on the stack, together with register values, transferring execution to an intermediate piece of code that identifies the location of the second piece of code, and then transferring execution to the second piece of code. A problem with transferring execution in this way is that a relatively large amount of time is spent in making the transfer.